The present invention concerns a method of processing a dried and decarbonated feed natural gas in order to obtain a natural gas intended to be liquefied and a cut of C5+ hydrocarbons, of the type comprising the following steps:                cooling of the feed natural gas in a first heat exchanger to form a pre-cooled feed flow;        introduction of the pre-cooled feed flow into a first separator flask to form a pre-cooled gaseous flow and optionally a pre-cooled liquid flow;        dynamic expansion of the pre-cooled gaseous flow in an expansion turbine and introduction of the expanded flow issuing from the expansion turbine into a first purification column;        optionally, expansion of the pre-cooled liquid flow and introduction into the first column;        production at the head of the first column of a purified head natural gas;        recovery at the bottom of the first column of a liquefied bottom natural gas;        introduction of the liquefied bottom natural gas into a second column for elimination of the C5+ hydrocarbons;        production, at the bottom of the second column, of the cut of C5+ hydrocarbons;        production, at the head of the second column, of a gaseous column head flow and introduction of the gaseous column head flow into a second separator flask to form a liquid bottom flux and a gaseous head flux;        introduction of a first part of the liquid bottom flux in reflux into the second column and introduction of a second part of the liquid bottom flux in reflux into the first column;        injection of at least a part of the gaseous head flux issuing from the second separator flask into the purified head natural gas to form the processed natural gas.        
Such a method is intended to be used in new natural gas processing units upstream of a liquefaction unit, or for modifying existing processing units in order to improve the safety thereof.
In particular, this method is adapted to the production of natural gas on floating platforms for production, storage and off-loading, designated by the acronym “FPSO” (“Floating Production Storage and Off-loading”). The FPSOs permit the production of natural gas from deep water fields, especially located in the bottom of expanses of water such as lakes, seas or oceans.
In order to facilitate the transport of natural gas extracted from a deposit, it is known to liquefy it in order to reduce its volume and permit its transport by ship. Before proceeding with liquefaction, the natural gas produced must be processed in order to eliminate a certain number of compounds.
In particular, the carbon dioxide content should be lowered to less than 50 ppm, the water content should be as low as possible and generally below 1 ppm and the content of sulphur-containing compounds, such as mercaptans, should be low. In particular, the concentration of hydrogen sulphide is generally fixed at less than 10 ppm and the concentration of other sulphur-containing compounds should be below 30 ppm.
In order to eliminate acid compounds, carbon dioxide and mercaptans, washing with a solvent such as an aqueous solution of amines is carried out. In order to eliminate water, molecular sieves are generally used.
In addition, the natural gas extracted generally contains a small amount of heavy hydrocarbon compounds, such as C5+ hydrocarbons like benzene.
The C5+ compounds must be eliminated from the natural gas before liquefaction in order not to cause blockages in the main gas liquefaction exchanger and in the equipment located downstream.
Conventionally, it is known for example to proceed with this elimination by cryogenic distillation. Such a distillation generally comprises steps of cooling of the feed gas in a series of exchangers using a propane type coolant and the feeding of a first fractionating column.
The method then comprises distillation in a column generally operating at a pressure of more than 40 bar, generally designated by the term “scrub column”.
Then the head of the column is partially condensed in an exchanger using a more volatile coolant than propane, before using a cascade method, in a series of three columns, to produce a gaseous mixture rich in C2, propane, butane and a cut of C5+ hydrocarbons.
Other processing methods are described for example in U.S. Pat. No. 7,010,937.
Such methods may be optimised thermally in order to obtain complete elimination of the C5+ hydrocarbons with good energy outputs.
However, these methods do not give complete satisfaction, particularly when they have to be carried out in a restricted space like that of a platform floating at sea, and/or in environments in which the safety constraints are high, such as, for example in urban areas.
In this respect, the use of hydrocarbon-containing coolants, such as propane or butane, and in particular the presence of storages for supplying these coolants, causes a safety problem, particularly taking into account their explosive nature.